1. Field of the Invention
This invention relates to catalysts which contain nickel and cobalt as active constituents in combination with MgO as promoter, and a carrier, as well as a process for their preparation. The catalysts are particularly suitable for hydrogenation carbon monoxide mainly into methane in a fluidized bed method.
2. Discussion of Prior Art
The shortage of natural gas in the foreseeable future has led to the development of various processes for the catalytic hydrogenation of carbon monoxide. Such processes are aimed at producing a synthetic gas (Substitute Natural Gas or SNG) which is largely equivalent to natural gas as regards composition and utilizability and is thus able to replace the latter. Natural gas frequently contains, in addition to methane, also ethane, propane or even butane in amounts of up to a total of about 0.1 to 5.0% by volume referred to the content of methane. These hydrocarbons raise the calorific value of the gas and are, therefore, desirable within certain limits. However, they effect the density, burning and combustion properties of the gas, with the result that their content must be kept within specific limits.
The processes for the catalytic hydrogenation of carbon monoxide to methane principally differ by the measures used to remove the heat of reaction liberated during the methanation. In most multi-stage processes the catalysts are arranged in a stationary system. Recent process developments use the fluidized bed technique for the methanation, since in this way particularly high space-time yields can be obtained and at the same time the thermal stress to which the catalyst is subjected is kept within bounds by virtue of the good heat transfer. Examples of the catalytic hydrogenation of carbon monoxide into mainly methane using the fluidized bed technique are given in German Offenlegungsschriften Nos. 2,449,587; 2,651,567 and 2,807,422.
The hydrogenation of carbon monoxide into mainly methane is catalyzed by nickel, ruthenium, cobalt or iron. Catalysts for technical requirements contain almost exclusively nickel as active main component, the nickel content normally amounting to 30 to 40% of the total weight of the catalysts. Cobalt is little used as active main component for methanation catalysts since, like iron, it has substantial disadvantages compared with nickel, the main disadvantage being the tendency to increased carbon deposition. The use of ruthenium, which possesses excellent properties for the hydrogenation of carbon oxides, has not hitherto been adopted on a technical scale.
In addition to hydrogenation-active metals, methanation catalysts normally contain additions of difficultly reducible oxides, which are used as electron or structural promoters, or also as carrier materials or constituents of carrier materials.
It is known to use nickel and cobalt together on carrier as a catalyst for hydrogenating carbon monoxide. Thus, the preparation of a catalyst preproduct containing nickel and cobalt oxides by precipitation is described in German Offenlegungsschrift No. 2 621 314. The catalyst obtained from the preproduct contains more than 40% by weight of nickel oxide and cobalt oxide and is used in the form of a catalyst fixed bed for the methanation.
German Offenlegungsschrift No. 2 631 901 concerns a catalyst for methanation according to the fixed bed method, which catalyst may also contain in addition to nickel as the active main component, cobalt, iron, copper, magnesium, zinc, aluminum and chromium. This catalyst is prepared by combined precipitation of the components as silicates, which have a composition comparable to that of natural minerals (serpentine). In addition to the metal silicates, this catalyst contains argillaceous minerals. The content of nickel or nickel and cobalt in the catalyst is 65% by weight. Details as to how and in what way the properties of the catalyst can be influenced by adding cobalt and/or magnesium are given in the publication.
Further processes for preparing nickel-containing methanation catalysts by precipitating the active components from aqueous solution in the presence of a carrier material or by combined precipitation of the active constituents and the carrier materials from their solution are described for example in German Offenlegungsschrift Nos. 2 231 316; 2 231 367 and 2 261 634.
A further possibility of applying catalytically active components for the methanation onto carrier materials is to impregnate carriers having a large surface area. Thus, U.S. Pat. No. 3,933,883 describes a methanation catalyst which contains a nickel-cobalt mixed oxide on extremely pure .gamma.-Al.sub.2 O.sub.3 obtained by impregnating the carrier material with a solution containing nickel and cobalt salts, followed by calcination. However, the catalyst has an insufficient catalytic activity and is unsuitable for fluidized bed methanation.
Catalysts for fluidized bed methanation have been described in the proceedings "Evaluation of Fluidized Bed Methanation Catalysts" of the "8th Synthetic Pipeline Gas Symposium" of the "American Gas Association", "Energy Research and Development Administration" and "International Gas Union" 1976 in Monroeville, Pa., as well as in German Offenlegungsschrift No. 2 449 587. According to the above, finely particulate catalysts consisting of mixtures of nickel oxide with oxides of chromium, molybdenum and tungsten or of cobalt with chromium, molybdenum and tungsten oxides on aluminum oxide as carrier are used for simultaneous fluidized bed conversion and methanation. The preparation of the catalysts is not described. Additionally, the active life of 25 to 26 days achieved with the specified catalysts is unsatisfactory.
German Offenlegungsschrift No. 2 816 035 describes a fluidized bed catalyst for producing synthetic natural gas. The catalyst is prepared by mechanically mixing nickel oxide with carrier material and hydraulic cement as binder in a moistened state, followed by compressing the composition into shaped bodies which are first of all thermally treated and then comminuted to a particle size of 40 to 350 .mu.m. The preparation of this catalyst is clearly very costly. With this catalyst too, a satisfactory active life cannot be obtained.
Special requirements are demanded of catalysts for fluidized bed methods. Additional requirements arise in the catalytic hydrogenation of carbon monoxide mainly resulting in methane. On account of the high gas velocities typical for fluidized bed reactions, resulting in short residence times of the reactants over the catalyst, the catalysts must have a high activity in order to ensure the rapid establishment of the reaction equilibrium.
In order to guarantee a satisfactory fluidization behaviour and thus a faultless technical operation, in addition specific requirements are placed on the mechanical properties of the catalyst, which principally concern its particle size distribution and density. An essential factor in evaluating a fluidized bed catalyst is also its mechanical resistance to abrasion and attrition. The amount of very fine material which has to be removed and extracted for operational reasons should be less than 1% by weight of the catalyst mass per operating day in order to keep the losses within tolerable limits (see e.g. A. Anderlohr, K. Hedden: GWF Gas, Erdgas 118, 422 (1977)).
For preparing methanation catalysts for fluidized bed methods, catalyst preparation by precipitation is disadvantageous for several reasons. The uniform deposition of the active components onto a carrier requires great care and is often difficult to reproduce. The catalyst precursor and the finished catalyst is usually obtained in a regular or irregular form, but not, however, in the particle size distribution required for use in fluidized bed methods. The measures adopted to obtain the desired particle size distribution inevitably result in an unavoidable incidence of fine and coarse particle fractions, which must be recycled to the production process or otherwise utilized, or considered as lost material. Considerable economic disadvantages thus result, especially in the case of catalysts containing large amounts of the expensive active components nickel and cobalt.
In order to achieve a technically and economically satisfactory operating time, the catalyst must also be sufficiently stable at high temperatures, i.e. roughly between 300.degree. and 600.degree. C.
The relatively high nickel and cobalt content of catalysts prepared by precipitation, amounting to roughly 30 to 60% by weight of the catalyst mass, leads to high costs when using these metals in methanation processes. A reduction in the concentrations of the active components nickel and cobalt to an economically more favorable level of e.g. less than 30% referred to the total catalyst results on the other hand in the case of precipitated catalysts in a marked drop in activity and useful operating time.
To summarize, it can thus be seen that a catalyst suitable for methanation of carbon monoxide in a fluidized bed method must in particular satisfy the following requirements:
It must have a high activity at the lowest possible concentration of the active components, and must also exhibit outstanding mechanical stability and satisfactory fluidization behaviour and be resistant to high temperatures. Its preparation should be simple and economical and should ensure the reproducibility of the desired catalyst properties.
Finally, the predominantly methane-containing gas mixture formed by hydrogenating carbon monoxide must be similar as regards composition to natural gas and should be largely free from unreacted carbon monoxide. In order to reduce the flame speed, an as low as possible hydrogen content is also desirable. A proportion of ethane, propane or butane in an overall range of 0.1 to 5% by vol. referred to methane raises the calorific value of the methane-rich gas and is thus desirable.